Surface treatment of tinned material



Patented Sept. 12, 1944 SURFACE TREATMENT OF TINNED MATERIAL Frank E. Rath, Bellevue, Pa.

No Drawing. Application January 14, 1942, Serial No. 426,789

5 Claims.

This invention relates to tinned material such as steel sheets and strip which have been provided with a coating of tin, and more particularly to the surface treatment thereof.

In the manufacture of tin-coated sheets and strip formed of steel, it is well known that the tin surface tends to' slightly discolor with a yellowishv cast. This discoloration is known as yellow stain, and ordinarily cannot be detected immediately after the tinning operation, but appears after the sheet or strip has been in storage for a substantial period of time, or upon exposure to elevated temperatures such as those required for baking during lithographing or enameling operations.-. Yellow stain does not render the sheet'or strip unsuitable for certain purposes such as for food containers, but is frequently objectionable from the standpoint of appearance. In addition, it sometimes interferes with forming operations, such as soldering, which is inherent in the manufacture of tinned containers.

It has been broadly accepted scientifically that yellow stain is caused by the presence on the surface of the tinned material of an oxide film,

and since the defect is not detectable immediatcly after tinning, the development of the film is progressive. In other words, oxidation of the tinned surface takes place following tinning operations, and an increasingly thick oxide film is developed on the surface, which film becomes visible, i. e., results in a yellow stain, when its thickness is sufficient to impart color to light reflected by the surface.

The first appearance of yellow stain is there-- fore intimately related to the rate of oxidation of the tinned surface, which rate will be markedly affected by the presence and nature of an initial film existing on the tinned surface, since the rate of oxidation, at any given temperature. depends on the facility with which oxygen reaches porous and permeable than those formed at a i,

low temperature, say, room temperature.

It is among the objects of the present invention to eliminate the tendency of tinned material, such as steel sheets and steel stripjto discolor to such an extent as to stain.

moval of any oil or grease on the tinned material.

fication.

In amanner well known to those skilled in the 1 H:

art, the tinned material such as steel sheets or strip is obtained by subjectin the same'to'the action .of a conventional tin pot the-structure" of which is believed so well knownltdthoseskmd I, .7. in the art as not to require illustration herein-I As the material to be coated, which will be desig hated hereinafter as the workpiece, progresses through the conventional tin pot, it penetrates a layer of flux, moves through a bath of molten tin, and leaves the bath of molten tin byv way of penetrating a lubricant on the surfaceof the molten tin, the said lubricant in most instances being palm oil. As the plate emerges from the tin pot, its temperature is near the melting point of tin and. I

its surface is covered with palm oil. In spite of the presence of the palm oil, the tin surface oxidizes. The oxidation is limited in normal ciently effective to preclude all oxidation; consequently the surface of the plate is covered by the tinned surface, which in turn depends on an imperceptible oxide film which, since it is formed at high temperature, is quite permeable, and as a result of this permeability the plate is subject to the subsequent appearance of yellow. stain. According to conventional methods, the workpiece is next subjected to the action ofa wet washer wherein it is immersed in water contain-.

ing 0.1 percent sodium carbonate (NazCOa).

The action of this solution in combination with the rolls in the wet washer which provide not only for motivation but also a slight scrubbing and squeegee action removes the excess lubricant (i. e.. palm oil or the like). However, some of the lubricant remains on the surface of the workpiece and the workpiece is next subject to the action of a branner wherein the surface is produce yellow tion on the surface.

scrubbed with bran, middlings, or the like. The action of the branner polishes the surface and regulates the lubricant film thereon.

' terial treated in the manner mentioned immediately hereinbei'ore (which, as before stated;

is entirely conventional) might accomplish the oil, resulting from hot dip methods of tin coating) but also results in the elimination of te'nd-- ency toward yellow stain.

In accordance with the teachings of the present invention, the workpiece to be tinned (whether it be in sheet or strip form) is fed through a conventional tinning machine or tii'r pot. As it projgresses through the tin pot, the workpiece is immersed influx on the entering side, then molten tin, and then a bath of palm oil on the exit side.

According to the present invention, as the plate leaves the palm oil, it is directed into and out of a vessel containing a hot causticsolution which includes an oxidizing agent. Disposed within this vessel are means of vigorously scrubbing the surface of the plate. .The scrubbingmeans may be, for example, differential speed cloth rolls or the like, or rotating brushes. The purpose of the scrubbing elementspis twofold: (1) 'to aid the solution in the removal of oil, and (2) more important, to activate the tinned surface and thus promote the action of the solu- I It should be understood that vigorous scrubbing of the plate surface is chromic acid, a strong oxidizing agent. It may contain from 0.1 to 5.0% (by weight) of caustic soda, NaOH. Dilute solutions, however, possess poor saponifying power and are not to be preferred. Concentrated solutions, on the other hand, are expensive and difficult to handle. For general purposes the use of 1.0% caustic soda has been found to be satisfactory.

absolutely essential to the success of the treatment. In fact, for the best results, the scrubbing should be sufficiently intense to; cause rec'ystallization of the tin surface. After several seconds of immersion and scrubbing in the hot solution, the plate is removed and rinsed with 1 water and dried. A hot water rinse is preferred since this facilitates the drying.

After treatment in the forementioned manner, it will be found that there has been removed from the surface of the tin plate all extraheous matter, and simultaneously therewith the solution will be found to have reacted with the surfacein such manner as to render it substi'intially immune to conditions which normally eaiuse yellow stain to appear.

In lieu of immersing the workpiece in the hot caustic solution including an oxidizing agent, the same may be sprayed thereon.

In any event, the vessel containing the solution or the means for spraying the same on the workpiece may be entirely conventional in design. That is to say, in case of immersion, a conventional wet washer from a tin stack may be utilizedor in the case of application. by sprays, any suitable spraying system may be used. It is essential, however,'that the treating equipment be so designed that contact of the tin plate with dissimilar metal does not occur, as otherwise the tin becomes etched and the surface accordingly impaired. It is under stood, of course, that scrubbing means are provided.

The treating solution contains two essential ingredients: caustic soda, a strong alkali; and

The chromic acid content of the solution should be at least two-tenths of the caustic soda concentration. It has been observed that less than this proportion accelerates rather than inhibits attack of the caustic on tin. Preferably the ratio of caustic soda to chromic acid should be maintained at 3 to 1. Larger quantities of the oxidizing agent can be used, but are not to be preferred. The effect of excess oxidizer is, however, neither deleterious nor be'neficial. r

' While the time. of treatment appears to be independent of'the concentration of the,solu-' tion, its temperature has a marked effect.

this reason it is preferred to operate at I".

or above, the maximum temperature being the boiling point of the solution. Not only is the efliciency of the treatment increased with increase in temperature, but there is the further advantage that the" drying of the workpiece after treatment is facilitated.

When using a not solution, a 5 -secohd period of immersion has been found. to produce satisfactory results. A slightly shorter time, however, is utilized if the solution is sprayed under pressure against the surface, the mechanical action of the sprays. aidingin removing palm: oil, etc. It will be observed, however, that longer exposures are not injurious. In lieu of caustic soda there may be used droxides of lithium, potassium, or any of the metals comprising group I, family A of the pe riodic tables, but, of course, the use of caustic soda is more economical. Similarly, other wellknown strong oxidizing agents may be used instead of chromic acid, for example,-an;y alkyl ine metal dichromate o'r permanganate, orhydrogeh peroxide. V

The treatment of the present invention not only results in plate which'is resistant toyellow stain but also plate which has a brighter, better appearance. The latter improvement results from the elimination of the conventional branher. The elimination of the branning' operation also reduces production costs considerably.

and removes a source of mechanical damage to the plate.

While I have described several specific era'- bodiments of the present invention, it will be seen that I do not wish to be limited, exactly thereto, since various modifications" may be made without departing from the scope of the invention as defined in the appended claims.

I claim: 1. The method of preventing formation of yellow stain" on tinned material havin'gitpe'r'e on a porous oxide filmwhich comprises vigorously scrubbing the tinned surface and applying thereto a hot caustic alkali solution containing an oxidizing agent.v

2. The method of preventing formation of yellow. f= aih on tinned material having there on a p5foiis oxide film which comprises vigor ously scrubbing the tinned surface and applying thereto a hot solution containing caustic soda and chromic acid.

3. The method, of preventing formation of thereto a hot solution containing from 0.1 to 5.0

per cent by weight) of caustic soda and chromic acid, the ratio of caustic soda to chromic acid being approximately 3:1.

5. The method of cleaning and preventing formation of yellow stain" on tinned material having thereon a porous oxide film which comprises vigorously scrubbing the tinned surface and applying thereto a hot caustic alkali solution containing an oxidizing agent.

FRANK E. RATH. 

